1. Field of the Invention
The invention relates to globe valves and is directed more particularly to a globe valve for high pressure power plant drain service, providing improved seat sealing and longer valve life.
2. Description of the Prior Art
High pressure drain valves in power plants operate under very severe conditions. The valves are subject to high pressure blowdown of steam that often is abrasive because of the presence in the steam of moisture droplets and solid particulates. Further, the valves operate with fluid medium typically of very high temperatures, on the order of 660.degree. F. in the case of saturated steam and 1020.degree. F. in the case of superheated stream. Pressures of 2300 psig, or thereabouts, are common, and pressures of 3800 psig, or more, are encountered in some super critical steam power plants. Such valves historically have been forged steel globe valves with integral hard-faced metal seats. In the operation of such valves, extreme variations in temperature often lead to "cooling" of the disk and valve stem, resulting in contraction of the stem. Such contraction often causes the disk to part from the seat, permitting fluid to bleed through the valve.
Further, in high temperature service, stress-relaxation of stems in closed valves may produce a loss of stem compression and similar bleeding past the seat.
Still further, when the seating surfaces have become damaged over time, the valves have simply been replaced. Periodic replacement of such valves generally has been an accepted requirement by high pressure steam power plant operators.
More recently, there have appeared globe valves having replaceable metal or ceramic disks and seats. However, experience has shown that replacement of an original disk and seat assembly with a new disk and seat assembly sometimes leads to diminution of sealing performance.
Accordingly, there is a need for a globe valve for use in high pressure steam power plant drain environments, which valves are provided with a facility for providing in the disk and seat, close and secure sealing, even with thermal contraction or stress relaxation of the valve stem. A further need exists for such valves having improved disks and seats, the disks and seats being formed of harder, more durable, metals or ceramics separately from the valve, and inserted into the valve during initial assembly, all in an economical manner. A still further need is for a globe valve as described above, the valve being adapted to receive replacement disks and seats, with the sealing performance of replacement disks and seats equaling the sealing performance of the original disk and seat.